What Controls the Water Vapor Isotopic Composition Near the Surface of Tropical Oceans? Results From an Analytical Model Constrained by Large-Eddy Simulations.
What Controls the Water Vapor Isotopic Composition Near the Surface of Tropical Oceans? Results From an Analytical Model Constrained by Large-Eddy Simulations.
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DOI：
10.1029/2020ms002106
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发表时间：
2020-08
影响因子：
6.8
通讯作者：
Blossey P
中科院分区：
文献类型：
--
作者：
Risi C;Muller C;Blossey P
这项研究的目的是了解控制热带海洋表面附近水蒸气同位素组成的机制，在大约一百公里和一个月的尺度上。在热带地区，人们长期以来一直观察到，当降水率高时，近地表的雨和蒸汽的同位素组成会更加贫化。这就是所谓的“数量效应”。以往的研究，基于观测或模式与参数化对流，强调了深对流和中尺度下沉气流和降雨蒸发的作用。但这些过程的相对重要性从未被量化。我们假设它可以使用受大涡模拟约束的分析模型进行量化。大涡模拟的结果证实，只有当降水率的变化是由大尺度环流的变化引起时，经典的量效应才能被模拟出来。我们发现，与海洋平衡相比，消耗水蒸气的主要过程是上升气流来自水蒸气更丰富的地区。造成数量效应的主要过程是，当大规模上升增加时，同位素垂直梯度更陡，因此上升气流和下降气流更有效地消耗云下层。只有当降水率较高时，降水才会更加枯竭，只有当它与更强的大尺度上升相关联时。上升气流是近地表水蒸气相对于与海洋平衡的大部分枯竭的原因。随着大尺度上升，近地表水蒸气更加枯竭，因为上升气流更有效地输出富集的水。
The goal of this study is to understand the mechanisms controlling the isotopic composition of the water vapor near the surface of tropical oceans, at the scale of about a hundred kilometers and a month. In the tropics, it has long been observed that the isotopic compositions of rain and vapor near the surface are more depleted when the precipitation rate is high. This is called the “amount effect.” Previous studies, based on observations or models with parameterized convection, have highlighted the roles of deep convective and mesoscale downdrafts and rain evaporation. But the relative importance of these processes has never been quantified. We hypothesize that it can be quantified using an analytical model constrained by large‐eddy simulations. Results from large‐eddy simulations confirm that the classical amount effect can be simulated only if precipitation rate changes result from changes in the large‐scale circulation. We find that the main process depleting the water vapor compared to the equilibrium with the ocean is the fact that updrafts stem from areas where the water vapor is more enriched. The main process responsible for the amount effect is the fact that when the large‐scale ascent increases, isotopic vertical gradients are steeper, so that updrafts and downdrafts deplete the subcloud layer more efficiently. The precipitation is more depleted when the precipitation rate is higher only if it is associated with stronger large‐scale ascent Updrafts are responsible for most of the depletion of the near‐surface water vapor relative to equilibrium with the ocean With large‐scale ascent, the near‐surface water vapor is more depleted because updrafts export enriched water more efficiently